THE THIEF

The thief is an unique leader who must act alone and can perform specific actions based on your LORD.

WARRIOR OR MAGE LORD
Poison an enemy party reducing their hit points;
Spy on the enemy troops or cities;
Engage an enemy party leader in a duel;
Steal spells from an enemy race or merchant; Steal items from an enemy party or a merchant.

GUILDMASTER
All of the above plus :
Assassinate the weakest unit in a party;
Counterfeit orders changing the battle position of an enemy party;
Provoke a riot in a city;
Steal gold from the city treasury;
Bribe by offering gold in return for a city allegiance.

A thief is always capture sooner of later and I find the result expensive.... if he is not stealing to make up for the expense naturally... Stealing potions and artifacts makes the game easier and less fun.

I encountered the city in flame in the Legion's saga. A thief provoked a riot. You can not enter your city while a "riot is taking place". After a few days, every thing is back to normal.

I rarely use a thief, a no! no! in the back of my head!!!, but the Clan's thief is the funniest of the whole units with his "I'm busy!", so I just leave him alone most of the time. I prefer to use a summoned creature to see what is in a ruins or or city and I rely most of the time on spells.

The AI is using thieves to block your path and I just hate it but the same trick works for you. I used a thief against a banshee in a duel once to save the life of my main leader. Use them only if you can not make it in an another way...